fix: handle missing timezone in activity title generation

Default title generation could fail when parsed activity data had no timezone set. This change adds a null/blank fallback in `ActivityFormatter` so titles can still be generated without throwing a `NullPointerException`.

Signed-off-by: Marcus Fihlon <marcus@fihlon.swiss>

src/main/java/net/javahippie/fitpub/util/ActivityFormatter.java

@@ -98,6 +98,10 @@ public class ActivityFormatter {
      *
      */
     private static LocalDateTime getUtcDateTimeInZone(LocalDateTime utcDateTime, String timezone) {
+        if (timezone == null || timezone.isBlank()) {
+            return utcDateTime;
+        }
+
         try {
             return utcDateTime.atZone(ZoneOffset.UTC)
                     .withZoneSameInstant(ZoneId.of(timezone))